PURPOSE: To reduce the rise of a dust discharging passage, to shorten the distance as well and to perform a strong dust collection with good efficiency, by composing it so as to open the air passage provided in a casing at the side part of a main shaft head, to face this opening into the hollow room communicated to a vacuum means and to reject therefrom.

CONSTITUTION: When a main shaft head A is operated with its movement from a retreat position to a work position, a valve mechanism D performs a release action and the other end opening (c) of the hollow room 40 inside is released. So, the chip generated in case of cutting reaches the hollow room 40 via the air passage (a) of the motor 25 outer peripheral part from the brush 28 inside and is rejected to the external part from a discharging port 44. Since the air passage (a) is open toward the side part of the main shaft head A and the hollow room 40 is corresponding to the opening (c) in the horizontal direction, there is no rise more than the necessity on the passage of the chip and the discharging passage can be composed in the shortest distance.